Particles Small Compared with the Wavelength

Note that the absorption cross section of a weakly absorbing sphere, like that of a particle small compared with the wavelength (Chapter 5), is, in the geometrical optics limit, proportional to its volume. This proportionality does not hold, however, for indefinitely large particle radius. Energy is absorbed primarily in the outer layer of a highly absorbing sphere (i.e., 2aa 1), and the interior of such a sphere plays no role in the absorption process. As the radius increases, therefore, the absorption cross section becomes proportional to area instead of volume. 

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